This is fallout from WT-2707. When building in develop I see two false positive messages from s_label:

s_all run of: "./s_label" resulted in:
    src/async/async_api.c:18: mix of returns and jump to the error label within a loop
    src/async/async_worker.c:124: mix of returns and jump to the error label within a loop

There is a single loop in each of those functions using TAILQ_FOREACH. Neither has a mix of returns or jumps to err that I can see.